1. Field of the Invention
The present invention relates to megasonic cleaning systems, and more particularly to a megasonic cleaning system for cleaning articles such as semiconductor wafers and photo masks used in semiconductor device fabricating processes.
2. Description of the Related Art
In general, the semiconductor device fabricating process comprises a series of sequential steps for forming desired patterns over the surface of semiconductor wafers, and then cleaning the wafer after each process step is completed in order to remove foreign particles and other residue on the surface of the wafer. The cleaning methods typically employ chemical and physical techniques to separate the foreign substances that are attached to the wafer surface.
Since ultra large scale integration (ULSI) technology predominates in the fabrication of semiconductor memory devices, the cleaning steps are increasingly important in removing extremely small particles after forming the minute patterns on the wafer or mask. By decreasing the design rule of the semiconductor memory device to submicron region, the tolerable size of the foreign particles produced during the fabricating process is decreased accordingly. The extremely small foreign particles may cause fatal defects during the fabricating process, thereby decreasing the reliability and yield of the fabrication facility.
However, it is difficult to remove the submicron-sized particles due to a strong adhesion force existing between the particles and the substrate. Some have tried to increase the cleaning efficiency by applying an external force to the particles sought to be removed, with the external force being sufficient to overcome the adhesion force of the particles. An example of such methods, incorporating ultrasonic or megasonic cleaning systems, are disclosed in "Mechanisms of Removal of Micron-Sized Particles by High Frequency Ultrasonic Waves," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 42, No. 4, July 1995. Also, U.S. Pat. No. 5,625,249 discloses a megasonic cleaning system including a tank containing cleaning fluid and articles to be cleaned, and piezoelectric transducers attached to the tank for transmitting acoustic energy at a frequency of approximately 1000 kHz into the cleaning tank.
A disadvantage of conventional megasonic cleaning methods is that they are wet methods, typically employing a chemical fluid and deionized water to conduct the cleaning. This results in a decrease in the cleaning efficiency due to the transmission of ultrasonic energy through the chemical medium, which causes particles contained in the chemical fluid to re-contaminate the article or device that is being cleaned. Further, the transmission of ultrasonic energy through the chemical medium may produce minute damages on the mask caused by the flow of the chemicals.